How to Calculate BAC
What is BAC?
Blood Alcohol Content (BAC) measures the percentage of alcohol in the bloodstream. It is calculated using the Widmark formula, which accounts for body weight, sex, number of drinks, and time elapsed. A BAC of 0.08% is the legal driving limit in most US states.
Step-by-Step Guide
- 1BAC = (drinks × 14g × 0.789) / (weight_kg × distribution_ratio × 1000) × 100
- 2Distribution ratio (r): 0.68 for men, 0.55 for women (alcohol distributes into body water)
- 3Subtract 0.015% per hour for alcohol metabolized by the liver
- 41 standard drink = 14g pure alcohol = 12oz beer / 5oz wine / 1.5oz spirits
Worked Examples
Input
160 lb male, 3 drinks in 2 hours
Result
BAC ≈ 0.05%
Below legal limit but impaired
Input
130 lb female, 3 drinks in 1 hour
Result
BAC ≈ 0.09%
Above legal limit in most jurisdictions
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